There is a need for sensitive detectors that can detect photon energy even in low-light conditions. Various applications utilize such detectors for low light imaging, laser communications applications, and applications such as LADAR where coherent lasers are employed instead of radio waves as in conventional RADAR. The carrier frequency of 1 um LADAR is about 3×1014 Hz, for example, and can provide about a 100,000 times improvement in spatial resolution over 1 GHz RADAR, for example. The promise of great improvement in spatial resolution has given impetus to the development of various LADAR applications. In addition to LADAR, other imaging applications also require detectors for very low light conditions.
In very low light applications, internal detector gain is required to boost the received photon signal above the noise floor of subsequent electronics and signal processing. For many years, the only device that provided such gain was the photomultiplier tube (PMT) based on vacuum tube technology. While offering high gain, the PMT has a number of practical limitations. Such limitations included dealing with a bulky vacuum tube, offering limited linearity, providing a narrow spectral response range, and providing a low quantum efficiency (QE) (typically <25%). The PMT also generates heat, requires several thousand volts for operation, and is not well suited for integration into system on chip (SOC) platforms. Also, long detector readout times are not optimal for fluorescent lifetime measurement. Various solid-state alternatives require several hundred volts for operation and have process limitations that are not compatible with standard semiconductor processing and integrated electronics.